This invention relates to an improved counterweight system for use with overhead doors. Counterweight systems for overhead doors are known in the art.
U.S. Pat. No. 5,103,890 discloses a system in which a cable is attached to a counterweight at one end. The other end the cable is mounted around a drum. The drum is carried by the shaft of the door. The drum has a progressive radius. When the cable is wrapped on the drum the conical shaped surface of the drum moves the cable outwardly during rotation thereby changing the counterweight torque. The purpose of changing the torque is to apply a greater force during opening of the door, when the effective weight of the door is the greatest. Conversely, when the effective weight is reduced, for example when the door is in its horizontal position, the cable is wrapped on the smaller diameter portion of the drum and the effective torque is reduced.